Premature gelling of 30-minute epoxy?

Epoxy 30 gives about 10 minutes working time not 30. (The epoxy sets in 30 minutes.)

On Mon, 05 Sep 2005 22:05:40 -0700, Harry Sanchez wrote in :

You may be able to get a few extra minutes of working time by mixing your epoxy on a piece paper rather than in the mixing cup.

The cup tends to hold the heat caused by the reaction of the two parts of the epoxy, causing it to set more quickly.

Of course, mixing a lot of epoxy on a flat surface isn't easy, either--there's a lot of potential for making a BIG mess. :o(

Maybe use a bigger cup or bowl?

Marty

You have maybe 15 minutes max working time. I set a timer so I know exactly how long I have to frantically spread epoxy and get pieces together. If you mix it on a flat surface instead of a cup, you will get a little longer working time. Do a trial mix and time how long it is before it starts to harden.

| I mixed about 10 ml of the epoxy ... | I tried to apply the epoxy to the root rib of the same wing half, | and within about 10 minutes after I had poured the two epoxy parts | into the cup, the epoxy started gelling and hardening, rendering it | useless.

30 minute epoxy seems to give around 10 minutes of good working time. Perhaps you need 2 hour epoxy, which is a bit stronger as well.

You can increase the amount of time you have to work with a few tricks --

- Mix less epoxy at once. As it cures, it heats up, which makes it cure faster, which makes it heat more, etc. Mixing less at once allows the heat to disappate better.

- Keep your epoxy in the fridge before mixing it. If it starts out cooler, it will take much longer to cure. (Chemical reactions happen slower if the temperature is reduced.)

The mixing cup is the problem, Harry.

Epoxy glue should be mixed on a flat surface. That way the heat that is generated during curing is transferred to the air and not held within the glue mass.

When I smoked, I used to cut up the cigarette cartons into disposable mixing pads. I still have a bunch of them left over, even though I quit smoking four years ago.

Another thing that works well are plastic lids from coffee cans and such.

Ed Cregger

I quit smoking

GOod job on that Ed. mk

Also, if you add some micro balloons, it will kick REALLY fast

especially if it is in a cup!

Jerr

-- tailskid

Been modeling since '49 - which makes me an Old Fart

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Too much epoxy in too deep a receptacle. Been there, done that, most of us have. Epoxy generates heat as it cures. Large amounts in a cup cannot disperse that heat so the stuff gells very quickly. Mix large amounts on a paper plate or something similar.

Could possibly have been mismarked. Also, the larger quantity of epoxy is mixed in a container, the quicker it can cure as it builds up heat. Was it hot where you were doing this? That also speeds the cureing.

I feed my two cats on paper saucers (?). But I never even thought about using them for mixing epoxy. An excellent suggestion, my friend.

Ed Cregger

If you use those plastic cups, and mix enough epoxy, you can actually melt the cup-even with 2 hr epoxy.

Had to toss a cup of 30 minute out the door into the yard once. Wasnt sure what the temp was going hit but it was sure as hell on the way.

Use to intentionally over temp polyester resin. We would put an ounce in one of those bathroom paper cups and squirt a bunch of catalyst in it. Stir and stand back. Did manage to set one cup on fire once.

It will go "all the way" if you mix in paper cups. If you have a small, deep container, it may not take much volume at all. **Never** toss a container of epoxy in the trash unless it's completely cured! We had the guys in the shop set the trash cans on fire at least once a year by tossing the remainder of a cup in the trash. They always make more than necessary as the ratio is more forgiving if you mix larger batches - the smaller the batch the more critical the measurements have to be, especially when you're not using the 1:1 stuff.

I also know guys that got their hands burned holding a paper cup as it cured. You don't really notice it because it's so gradual - at first. By the time it starts to hurt, you're already on the way to blisters! Gotta be real careful with the stuff, it can get you in several ways. Cheers, jc

Harry,

Ed Cregger said just about everything I was going to say. Check ou the FAQ on the BSI website and you will see this is the #2 question. keep using the same plastic coffee can top to mix my epoxy. The cure epoxy will pop right off when you flex the plastic top and you ca start all over. Laying down equal length parallel beads of the tw parts is the method I prefer. At 75 degree F, you will get 30 minute working time. For every degree above that, you loose about one minute Some other companies do rate their 30 minutes as a cure time and not working time. The working time for the BSI 5, 15, & 30 minute epoxie will be just that if they are mixed on a flat surface at about 7 degrees. BSI's Finish-Cure 20 minute epoxy will actually give you a 4 minute working time on a flat surface, but since it is usually mixed i quantities of one ounce or more, the rating of 20 minutes is given fo the amount of time it will kick off in a cup.

I do not recommend Doug's idea of keeping the epoxy in the fridge. First, it will become very thick and hard to get out of the bottle. Second, if the epoxy is not warmed to above 70 degrees before applying it will never cure correctly, if at all. 5 minute epoxy doesn't hav this problem, however: it generates enough heat by itself.

Charlee Smith Bob Smith Industrie

-- Charle

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| I do not recommend Doug's idea of keeping the epoxy in the fridge. | First, it will become very thick and hard to get out of the bottle.

Never really had a problem with that.

| Second, if the epoxy is not warmed to above 70 degrees before applying, | it will never cure correctly, if at all.

It still needs to be mixed properly, and if it's thicker than usual, it will be harder to mix, so you'll need to work at it a bit more. That may have been your problem if you tried it and it didn't cure.

After all, having it at 40 degrees at first just buys you a few extra minutes -- eventually (probably in 15 minutes or so), it'll be back at room temperature. Now, if it's already been applied and was never properly mixed, then it's never going to cure properly. But if it was properly mixed, it should already be on it's way to curing properly.

| 5 minute epoxy doesn't have this problem, however: it generates | enough heat by itself.

Personally, I prefer two hour epoxy -- I hate being rushed. The only time I use five minute epoxy is for something that needs to be held in a specific position until it cures and it's something that I need to be physically holding in my hands -- and even then I'd rather have 15 minutes to wait.

Epoxy is regularly mixed and frozen in the aerospace industry. It is then brought up to room temp and applied. Most of these require an oven cure, though.

I am sure it would work with the 2 hour variety if you could freeze it quickly enough. Working time after it was warmed might not be very long.

"Charlee Smith

Bob Smith Industries"

Glad to see you have joined us Charlee.....Your products ar excellent!

Jerr

-- tailskid

Been modeling since '49 - which makes me an Old Fart

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One way round this problem is to mix the epoxy on one of the surfaces to be joined. With practice one can estimate the amount needed, mix this on one part and then stick the two together - saves a lot of waste.

In the 1960s I used a big range of epoxy resins which had to be mixed by weight. Some of the catalysts were of water consistency so some sort of cup had to be used. We never had "premature gelling", but then most of them took hours to cure unless the temperature was raised artificially. Some 24 hour cures were reduced to about 1 hour at 60 deg C or a few minutes at, from memory, 120 deg C. We used to put many components into an oven, and the literature stated that higher temperature cures gave better bond strength.

Malcolm